Solder paste

ABSTRACT

A solder paste comprising a powder of a Zn-containing Sn-based, lead-free solder mixed with an activator-containing rosin-based flux to which 0.5-10.0 wt % of isocyanuric acid or a haloalkyl ester thereof is added can prevent the formation of solder balls and voids during reflow soldering and exhibit good solderability. A solder paste comprising a powder of an Ag- or Zn-containing Sn-based, lead-free solder mixed with an activator-containing rosin-based flux to which 0.01-10.0 wt % of a salicylamide compound also does not exhibit a change in viscosity and exhibits good solderability.

TECHNICAL FIELD

This invention relates to a solder paste for use in soldering ofelectronic devices and in particular to a solder paste of an Sn-based,lead-free solder.

BACKGROUND ART

Sn—Pb alloys have continued to be used as solder since ancient times.They have advantages of low melting points and good solderability. Forexample, a eutectic Sn63%-Pb37% solder, the most popular solder, has amelting point of 183° C.

The eutectic Sn—Pb solder has also been commonly used in soldering ofelectronic devices. Recently, surface mount technology (SMT) has beenemployed increasingly to mount electronic parts on a printed board dueto the capability of SMT to reduce size, enhance packaging density andperformance, and lower operating costs. In SMT, soldering is typicallycarried out by the reflow soldering method using a solder paste (alsocalled a cream solder) which comprises a solder powder uniformly mixedwith a soldering flux, particularly a rosin-based flux. In general, thereflow soldering method comprises feeding a solder paste onto a printedboard by printing or dispensing (discharging through a dispenser),temporarily fixing chip-type electronic parts on the solder paste by useof the adhesion of the paste, and heating the printed board with theelectronic parts thereon in a reflow furnace to allow the solder tomelt, whereby the parts are fixed and electrically connected to theprinted board.

Printed boards withdrawn from waste electronic devices by disassemblyare frequently discarded by pulverizing in a shredder and burying in theground. When rain of recent years which has been acidified (i.e., acidrain) contacts the pulverized printed boards buried in the ground, thelead (Pb) in the Sn—Pb solder may be dissolved out and contaminateunderground water. If a human or animal continues to drink alead-containing water for long years, there is the concern that lead isaccumulated in its body to cause lead poisoning. Therefore, it has beenrecommended in the art to use a lead-free solder, which is completelyfree from lead, in soldering of electronic devices.

Lead-free solders should consists of element which are harmless tohumans. For example, Cd should not be used, even though it has an effectof lowering a melting temperature. Lead-free solders which are promisingat present are Sn-based alloys which comprise a major proportion of Snand one or more alloying elements such as Ag, Cu, Bi, In, Sb, or Zn.

Among these, Ag-containing Sn-based solders (hereunder referred to asSn—Ag based solders) such as an Sn—Ag alloy and an Sn—Ag—Cu alloy havethe advantage of being easy to handle since they have relatively goodwettability as a lead-free solder. However, Sn—Ag based, lead-freesolders have a melting point on the order of 220° C., which isapproximately 30-40° C. higher than that of a eutectic Sn—Pb alloy, sothe working temperature (heating temperature in soldering) also becomeshigher correspondingly. Therefore, they may not be suitable for use insoldering of some thermally sensitive electronic parts.

Zn-containing Sn-based solders (hereunder referred to as Sn—Zn basedsolders) are advantageous from the standpoints of safety and economy,since Zn is not only an element which is harmless and indispensable forthe human body but is found underground in large amounts so that itscost is low compared to Ag, Cu, Bi, In, and the like. A typical alloycomposition of an Sn—Zn lead-free solder is Sn-9Zn. This solder has amelting point of 199° C., which is approximately 20° C. lower than thatof an Sn—Ag lead-free solder, so it also has the advantage that it canbe used for soldering thermally sensitive electronic parts to which anSn—Ag based, lead-free solder cannot be applied.

In a lead-free solder paste comprising a powder of an Sn-based,lead-free solder including an Sn—Ag and Sn—Zn based solder (such solderpaste being hereunder referred to as Sn-based, lead-free solder paste)mixed with a rosin-based flux, a reaction between the solder powder witha constituent of the flux, particularly with an activator which isnormally present in the rosin-based flux, may occur readily due to thehigh content of the active Sn metal in the solder powder, compared to aconventional solder paste prepared from an Sn—Pb solder powder and thesame rosin-based flux. Such a reaction may result in a change inviscosity of the solder paste and cause the problem that the solderpaste cannot satisfactorily be fed by printing or dispensing.

As a countermeasure, an approach can be employed, particularly in asolder paste of an Sn—Ag based solder, in which the viscosity change iscontrolled by decreasing the amount of an activator, e.g., an aminehydrohalide and organic acid, added to the rosin-based flux, therebysuppressing a reaction of the flux with the solder powder. However,since this approach weakens the activity of the flux, it may cause theformation of solder balls and adversely affect the wettability of thesolder. Accordingly, there is a need to control the viscosity change ofan Sn-based, lead-free solder paste without decreasing the amount of anactivator in the flux.

In addition to the above-described problem, a solder paste comprising apowder of an Sn—Zn based, lead-free solder (hereunder referred to as anSn—Zn based solder paste) has another problem. Zn is a metal which issusceptible to oxidation due to its high ionization tendency. Therefore,an oxide layer is formed on the surface of the solder powder whichcontacts air, and it makes the wettability of the solder powder poor. Inparticular, in an Sn—Zn based solder paste prepared by use of arosin-based flux, the surface oxidation of the solder powder by areaction with the flux becomes even more severe, so soldering defectsincluding formation of voids due to extremely poor wettability of thesolder and formation of solder balls may occur frequently.

In order to increase the wettability of the solder in the Sn—Zn basedsolder paste, it is conceivable to use a rosin-based flux containing anincreased amount of an activator. However, as described above, theactivator tends to react with the solder powder prior to soldering, andan increase in the amount of an activator may cause the viscosity of thesolder paste to increase rapidly and interfere with the feeding of thesolder paste by printing or dispensing.

In an Sn—Zn-based solder paste, an approach in which the surface of theSn—Zn based solder powder is coated with a suitable material prior tomixing with the flux to prevent the solder powder from reacting with theflux and causing surface oxidation may be employed. As the coatingmaterial, a precious metal such as Au or Pd, an inorganic oxide formedfrom a hydrolyzable organosilicon compound or the like, or an organicsubstance such as an imidazole or a triazole can be used.

However, such coating adds to the manufacturing costs of a solder paste.Furthermore, some types and methods of coating may promote oxidation ofsolder powder during the coating operation, and the method is notnecessarily effective at improving solder wettability or solderability.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to improve the solderability ofa Sn-based, lead-free solder paste comprising a powder of an Sn-based,lead-free solder mixed with a rosin-based flux, particularly onecontaining an activator.

Another object of the present invention is to provide such an Sn-based,lead-free solder paste exhibiting a minimized viscosity change andhaving good wettability.

A further object of the present invention is to provide an Sn-based,lead-free solder paste, and in particular an Sn—Zn based solder paste,which has good solder wettability and solderability in which the surfaceoxidation of the solder powder which is encountered significantly with aconventional Sn—Zn based solder paste can be prevented without coatingof the solder powder.

In one aspect, the present invention provides a solder paste comprisinga powder of an Sn-based, lead-free solder mixed with a rosin-based flux,wherein the rosin-based flux contains 0.5-10.0 wt % of at least oneisocyanuric compound selected from isocyanuric acid and derivativesthereof having no hydroxyl group.

In another aspect, the present invention provides a solder pastecomprising a powder of an Sn-based, lead-free solder mixed with arosin-based flux, wherein the rosin-based flux contains 0.01-10.0 wt %of at least one salicylamide compound selected from salicylamide andderivatives thereof.

DETAILED DESCRIPTION OF THE INVENTION

A rosin-based flux for use in a solder paste is normally prepared bydissolving a rosin in a solvent along with one or more additives such asan activator and a thixotropic agent. In particular, the type and amountof the activator have a great effect on the solder wettability of thesolder paste.

In general, an Sn-based, lead-free solder has poor solder wettabilitycompared to an Sn—Pb solder. For example, compared to the spreadingfactor of an Sn—Pb solder, an Sn—Ag lead-free solder spreads only about80% as much as an Sn—Pb solder. An Sn—Zn lead-free solder has an evenworse spreading factor, and it only spreads about 70% as much as anSn—Pb solder.

Therefore, it seems to be advantageous for an Sn-based, lead-free solderpaste to incorporate a large amount of an activator to improve thewettability. However, as described previously, this causes the viscosityof the solder paste to increase rapidly due to a reaction between theactivator and the solder powder. As a result, the solder paste soonbecomes difficult to be fed by printing or through a dispenser, and itsshelf or service life becomes extremely short. Therefore, unless thesolder powder is protected by surface coating, the amount of theactivator should be decreased, resulting in poor wettability. Inparticular, a conventional Sn—Zn based, lead-free solder paste has verypoor wettability.

In reflow soldering using a solder paste, soldering is performed in aheating furnace called a reflow furnace. Heating in a reflow furnace isnormally two-step heating in which preheating is first carried out at atemperature of 150-170° C. for 30-100 seconds followed by main heatingat a temperature of 20-50° C. above the melting temperature of thesolder. Preheating is carried out in order to vaporize the solvent inthe solder paste and to simultaneously alleviate heat shock toelectronic parts to be soldered.

Materials for use in a solder paste of a eutectic Sn—Pb solder, which isstill most popular at present, are designed so as to exhibit optimalperformance in this two-step heating. For example, an activator beginsto react partially from around 150° C., which is the preheatingtemperature, and completely reacts at around 210-230° C., which is themain heating temperature for an Sn—Pb solder. In a solder paste of anSn—Pb solder, even if the activator used becomes active at a temperatureas low as 150° C., oxidation of the solder powder does not occursignificantly during the preheating stage due to the low reactivity ofPb.

In contrast, it was found that with an Sn—Zn based, lead-free solderpaste, the zinc exposed on the surface of the Sn—Zn based solder powdercan readily combine with the oxygen present in the reflow furnace toform a relatively thick surface oxide layer on the solder powder, evenduring the preheating stage. If the solder powder having such an oxidelayer formed by oxidation during preheating is subjected to mainheating, the solder powder melts with the oxide layer covering itssurface, so it remains without spreading as solder balls. In addition,the flux is mostly consumed by reacting with the surface oxide layer onthe solder powder and loses its activity, resulting in poor wettabilityand causing the formation of voids in the interior of the resultingsoldered joints. The formation of these solder balls and voids becomes acause of soldering defects.

As already stated, due to the high ionization tendency andsusceptibility to oxidation of Zn, an Sn—Zn based, lead-free solder ishighly susceptible to surface oxidation. In particular, it isconjectured that with a solder paste, the solder is in the form of apowder having an extremely large surface area, so oxidation of the Sn—Znbased solder becomes severe during the preheating stage, resulting in amarkedly adverse effect on solderability.

The oxidation of an Sn—Zn based, lead-free solder powder during thepreheating stage can be prevented if oxygen is nearly completely removedfrom the atmosphere in the furnace, as is achieved in an N₂ reflowfurnace. However, an N₂ reflow furnace is expensive and has high runningcosts.

It has been found that the oxidation of an Sn—Zn based, lead-free solderpowder during preheating can be prevented by adding at least oneisocyanuric compound selected from isocyanuric acid and derivativesthereof having no hydroxyl group to a rosin-based flux used to mix withthe solder powder to prepare a solder paste, according to the presentinvention. Although it is not intended to be bound by a specific theory,the reason therefor is presumed to be as follows.

Isocyanuric compounds generally have high heat resistance, and they donot undergo decomposition during heating at 150-170° C. for 30-100seconds, which are the standard preheating conditions in a reflowfurnace. Therefore, if a flux which contains an isocyanuric compound isused to prepare an Sn—Zn based solder paste, the isocyanuric compoundwill cover the surface of the Sn—Zn-based solder powder duringpreheating in a reflow furnace, and surface oxidation thereof can beeffectively prevented.

One of the reasons for using an isocyanuric compound having no hydroxylgroup is as follows. The rosin-based flux used to prepare a solder pastecontains an activator in order to improve wettability. The activatorusually comprises a hydrohalide salt of an organic amine. In thepresence of a polar substance, the amine hydrohalide can react with theSn—Zn based solder powder prior to soldering, particularly at the timeof printing when the solder paste is exposed to air. Due to thisreaction, the viscosity of the solder paste changes and usuallyincreases, thereby making it difficult to perform printing smoothly.This tendency is particularly strong with an Sn—Zn lead-free solderpaste, so the use of a polar substance should be avoided. Sinceisocyanuric acid derivatives having a hydroxyl group exhibit polarity,they are not used in a flux for a solder paste according to the presentinvention.

If an isocyanuric acid derivative having a hydroxyl group is used in aflux for an Sn—Zn based, lead-free solder paste, the zinc on the powdersurface may be oxidized to cause the surface to turn white duringwashing with water, which is performed subsequent to soldering. This isanother reason why an isocyanuric acid derivative having a hydroxylgroup is not suitable for an Sn—Zn based, lead-free solder paste.

Examples of isocyanuric compounds which are suitable for use in thepresent invention are isocyanuric acid and derivatives thereof havingthe following formula:

where R₁-R₃, which may be the same or different from each other, standfor hydrogen, an alkyl group optionally substituted with a halogen, oran allyl group. An alkyl group is preferably a lower alkyl group having1 to 6 and more preferably 1 to 4 carbon atoms. A compound in whichR₁-R₃ are all hydrogen is isocyanuric acid. A compound in which they areall alkyl groups is an isocyanuric acid trialkyl ester.

Specific examples of such isocyanuric compounds include isocyanuricacid, trimethyl isocyanurate, triethyl isocyanurate, tripropylisocyanurate, butyl isocyanurate, tris(2,3-dibromopropyl) isocyanurate,triallyl isocyanurate, and the like. One or more isocyanuric compoundsmay be used.

Isocyanuric compounds which are preferably used in the present inventionare isocyanuric acid and an isocyanuric acid tris(haloalkyl) ester (acompound of the above formula in which R₁-R₃ are haloalkyl groups) suchas tris(2,3-dibromopropyl) isocyanurate. Both of these may be usedtogether. In particular, it is preferable to use isocyanuric acid byitself or as a mixture with another isocyanuric compound.

The amount of the isocyanuric compound which is present in therosin-based flux is in the range of 0.5-10.0 wt %. If this amount issmaller than 0.5 wt %, the effect of preventing surface oxidation ofSn—Zn-based solder powder during preheating is not substantiallyachieved, while if more than 10.0 wt % is added, solderability isimpeded. The amount of an isocyanuric compound which is added to theflux is preferably 2-6 wt %.

In accordance with another aspect of the present invention, a solderpaste comprises an Sn-based, lead-free solder powder and a rosin-basedflux which contains at least one salicylamide compound selected fromsalicylamide (=salicylic acid amide) and derivatives thereof in anamount of 0.01-10.0 wt %.

Although it is not intended to be bound by a specific theory, it ispresumed that the salicylamide compound is preferentially adsorbed bythe surface of the solder powder so that it can prevent the solderpowder from reacting with a constituent of the flux, particularly anactivator such as an amine hydrohalide and organic acid. As a result, achange (mainly an increase) in the viscosity of the solder paste causedby such a reaction is prevented and the solder paste can be smoothly fedby printing or dispensing even after storage for a prolonged period. Ithas also been found that the salicylamide compounds do not interferewith the reflow properties of the solder paste.

The effect of a salicylamide compound in suppressing a change inviscosity of a solder paste is observed with a solder paste containing apowder of any Sn-based, lead-free solder including an Sn—Ag based andSn—Zn based solder.

The salicylamide compound useful in the present invention includessalicylamide, decamethylenedicarboxylic acid disalicyloyl hydrazide,3-(N-salicyloyl)-amino-1,2,4-triazole, and the like. If the amount of asalicylamide compound present in the rosin-based flux is less than 0.01wt %, the above-described effect is not appreciably attained. If it ismore than 10.0 wt %, melting of the solder powder may be inhibited,resulting in the formation of solder balls during soldering. The amountof the salicylamide compound in the flux is preferably 0.1-3.0 wt %.

As set forth previously, a salicylamide compound seems to exert itseffect of suppressing the viscosity change of an Sn-based, lead-freesolder paste through adsorption by the surface of the Sn-based,lead-free solder powder in the solder paste. Thus, instead of adding thesalicylamide compound to the flux, it is also possible for thesalicylamide compound to be previously adsorbed by the surface of thesolder powder, using surface treatment of the solder powder by, forexample, spraying with or immersion in a solution of the salicylamidecompound dissolved in an appropriate volatile solvent, followed byevaporation of the solvent. The resulting Sn-based solder powder havingthe salicylamide compound deposited thereon by adsorption may be mixedwith a rosin-based flux which does not contain a salicylamide compoundto prepare a solder paste.

Thus, the present invention also provides a process for preparing asolder paste comprising subjecting an Sn-based, lead-free solder powderto surface treatment with a solution of a salicylamide compound in anorganic solvent, and mixing the resulting solder powder having thesalicylamide compound deposited thereon with a rosin-based flux.

In this process, the surface treatment is preferably performed such thatthe amount of the salicylamide compound deposited on the solder powderis in the range of 0.01-10.0 wt % based on the weight of the flux to bemixed with the surface-treated solder powder in the next step.

A flux used in a solder paste according to the present invention is arosin-based flux having a rosin as a primary constituent. Typically, arosin-based flux comprises a rosin, an activator, a thixotropic agent,and a solvent. Except for the addition of an isocyanuric compound or asalicylamide compound to the flux according to the present invention,the type and amount of the other constituents of the flux may be thesame as for a conventional rosin-based flux, and there are no particularrestrictions. It is also possible for a solder paste according to thepresent invention that the flux contains both an isocyanuric compoundand a salicylamide compound each in an amount in the above-describedrange, thereby further improving the solderability of the paste.

The rosin may be a natural, unmodified rosin such as a gum rosin, a talloil rosin, or a wood rosin, or it may be a modified rosin such as apolymerized rosin, a hydrogenated rosin, a rosin ester, orrosin-modified resin. Of course, two or more of these may be used.

As an activator, it is preferable to use a hydrohalide and particularlya hydrobromide of an organic amine. The organic amine may be primary(such as an ethyl amine), secondary (such as a diethyl amine), ortertiary (such as a triethyl amine). A heterocyclic amine such aspyridine, an aromatic amine such as aniline, an alicyclic amine such ascyclohexylamine, and a compound having two or more amino groups such asdiphenylguanidine may also be used.

An organic acid activator such as stearic acid and sebacic acid oranother activator may also be used, preferably in addition to an aminehydrohalide activator.

Hardened castor oil, an amide, or the like is generally used as athixotropic agent.

Examples of a solvent are Carbitols such as butyl Carbitol and hexylCarbitol, and alcohols such as terpineol and halogenated alcohols.

The amounts of the above-described constituents in the flux are, forexample, 35-60% for the rosin, 0.5-10% for the activator, 1-10% for thethixotropic agent in weight percent. The amount of an amine hydrohalideactivator is preferably 0.5-5%. Of course, the flux also contains anisocyanuric compound and/or a salicylamide compound according to thepresent invention. The flux may contain one or more additives other thanthose mentioned above.

A powder of an Sn-based, lead-free solder used in a solder pasteaccording to the present invention may be a powder of an alloy comprisedpredominantly of Sn and including one or more of the elements Ag, Cu,In, Sb, and Zn. Preferably, the Sn-based, lead-free solder is an Sn—Agalloy (e.g., Sn-3.5Ag), an Sn—Ag—Cu alloy (e.g., Sn-3Ag-0.5Cu), an Sn—Znalloy (e.g., Sn-9Zn), or an Sn—Zn—Bi alloy (e.g., Sn-8Zn-3Bi), in whichthe numeral preceding an element indicates the content of the element inweight percent. These Sn-based, lead-free solders may further includeone or more of Bi, In, Ag, Ni, Co, Mo, Fe, P, Ge, Ga, or the like withthe object of further lowering the melting point, increasing themechanical strength, or suppressing oxidation of the solder.

In the embodiment of the present invention in which the flux contains anisocyanuric compound, the Sn-based, lead-free solder is preferably anSn—Zn based solder such as an Sn—Zn alloy or Sn—Zn—Bi alloy. Asdescribed previously, the isocyanuric compound present in the flux ishighly effective in preventing a solder powder from oxidizing at thepreheating stage in a reflow furnace. Thus, when the embodiment isapplied to a solder paste comprising a powder of an Sn—Zn based,lead-free solder which is highly susceptible to oxidation, aparticularly significant effect can be obtained.

On the other hand, the second embodiment of the present invention inwhich the flux contains a salicylamide compound can satisfactorily beapplied to a solder paste comprising a powder of any Sn-based, lead-freesolder including an Sn—Ag based alloy and an Sn—Zn based alloy, althoughits effect with an Sn—Zn solder is generally greater than with an Sn—Agsolder.

The shape of the solder powder is not critical, but normally it is aspherical powder which can be prepared by the centrifugal atomizingmethod, the gas atomizing method, or the like. The particle size of thesolder powder may be the same as for a conventional solder paste, and itis normally on the order of 200-400 mesh, but it is also possible to usea powder which is 500 mesh or finer.

The mixing ratio of the solder powder and the flux may be selected so asto obtain a solder paste having a viscosity suitable for printing ordispensing. Normally, the flux is 5-20 wt %, and the remainder is asolder powder.

The following examples are presented to further illustrate the presentinvention. These examples are in all respects illustrative and are notintended to be restrictive. In the examples, percentage indicates weightpercent unless otherwise indicated.

EXAMPLES Examples 1-3 and Comparative Example 1

88.5% of a spherical powder of an Sn—Zn based, lead-free solder alloyhaving a composition of 8% Zn, 3% Bi and a remainder of Sn wasthoroughly mixed with 11.5% of a rosin-based flux having the compositionshown in Table 1 to prepare a solder paste.

Using the solder paste prepared above, a reflow test was carried out ina standard reflow furnace of a two-stage heating type in whichpreheating and main heating were performed under usual conditions, andsolderability (wettability) and the formation of solder balls wereevaluated. The results are also shown in Table 1.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example 1 FluxComposition (wt %) Polymerized rosin 44 41 41 46 Diphenylguanidine 2 2 22 HBr Hardened castor oil 5 5 5 5 Isocyanuric acid 2 — 1 — Tris(2,3- — 54 — dibromopropyl) isocyanurate 2,3-Dibromo-1- 2 2 2 2 propanolAlpha-terpineol 45 45 45 45 Test results Solder balls none a few nonemany Solderability excellent good excellent poor

As can be seen from the results in Table 1, when a solder paste of anSn—Zn based solder which was prepared using a flux containing anisocyanuric compound according to the present invention was used forreflow soldering in air under standard conditions, it was clearlysuperior with respect to the amount of formation of solder balls andsolderability compared to the solder paste of Comparative Example 1 inwhich an isocyanuric compound was not added to the flux.

Examples 4-5 and Comparative Examples 2-3

88.5% of a spherical powder of an Sn—Zn based, lead-free solder alloyhaving a composition of 8% Zn, 3% Bi, and a remainder of Sn or of anSn—Ag based, lead-free solder having a composition of 3% Ag, 0.5% Cu,and a remainder of Sn was thoroughly mixed with 11.5% of a rosin-basedflux having the composition shown in Table 2 to prepare a solder paste.

The solder paste prepared above was allowed to stand at room temperatureto determine the shelf life, which is the period that the solder pasteretains a pasty state capable of printing or dispensing, as an index toevaluate the speed of viscosity change of the solder paste. The resultsare also shown in Table 2 with the type of solder powder.

TABLE 2 Comparative Comparative Example 4 Example 5 Example 2 Example 3Flux Composition (wt %) Polymerized rosin 49 47 50 49 DiphenylguanidineHBr 1 1 1 1 Stearic acid 5 5 5 5 Hardened castor oil 5 5 5 5Decamethylenedicarboxylic 1 — — — acid disalicyloyl hydrazide3-(N-salicyloyl)- — 2 — — amino-1,2,4-triazole 2,3-Dibromo-1-propanol —2 — 2 Alpha-terpineol 38 38 38 38 Solder powder/Test Result SolderPowder Sn—8Zn—3Bi Sn—3Ag—0.5Cu Sn—8Zn—3Bi Sn—3Ag—0.5Cu Shelf Life >12weeks >12 weeks 1 week 6 weeks

As can be seen from Table 2, conventional lead-free solder pastes ofComparative Examples 2 and 3 in which the flux did not contain asalicylamide compound lost a pasty state and became unsuitable for useafter 6 weeks for the Sn—Ag based solder and even after 1 week for themore reactive Sn—Zn based solder. In contrast, the addition of asalicylamide compound to the flux according to the present inventioncould extend the shelf life of the solder paste by a factor of at least2 for the Sn—Ag based solder and by a factor of at least 6 for the Sn—Znbased solder, indicating that it could effectively suppress a viscositychange of an Sn-based, lead-free solder paste.

INDUSTRIAL APPLICABILITY

The present invention provides an Sn-based, lead-free solder paste whichhas improved solderability and which can be satisfactorily used in aconventional reflow soldering method. As a result, in the case of, forexample, a solder paste comprising a powder of an Sn—Zn based, lead-freesolder which is highly susceptible to oxidation, it is possible toprovide a solder paste in which formation of solder balls is suppressedand which has excellent wettability using a flux containing a properamount of an activator as is conventional. It is also possible tosuppress a change in viscosity of an Sn-based, lead-free solder pastecomprising a powder of an Sn-based solder including an Sn—Ag based andSn—Zn based solder, thereby making it possible to use such lead-freesolder paste for a significantly prolonged period while making printingor dispensing operations easy.

Although the present invention has been described particularly withrespect to preferred embodiments, they are merely illustrative and donot limit the present invention. It should be understood by thoseskilled in the art that various modifications can be made on theembodiments described above without departing from the scope of thepresent invention as set forth in the claims.

1. A solder paste comprising a powder of an Sn-based, lead-free soldermixed with a rosin-based flux, the rosin-based flux containing 0.01-10.0wt % of at least one salicylamide compound selected from salicylamideand derivatives thereof.
 2. A solder paste as claimed in claim 1 whereinthe lead-free solder is selected from an Ag-containing Sn-based solderand a Zn-containing Sn-based solder.
 3. A solder paste as claimed inclaim 2 wherein the Zn-containing Sn-based solder further contains Bi.4. A solder paste as claimed in claim 2 wherein the Ag-containingSn-based solder further contains Cu.
 5. A solder paste as claimed inclaim 1 wherein the salicylamide compound is selected from salicylamide,decamethylenedicarboxylic acid disalicyloyl hydrazide, and3-(N-salicyloyl)-amino-1, 2, 4-triazole.
 6. A solder paste as claimed inclaim 1, wherein the rosin-based flux further contains 0.5-10.0 wt % ofat least one isocyanuric compound selected from isocyanuric acid andderivative thereof having no hydroxyl group.
 7. A solder paste asclaimed in claim 6 wherein the lead-free solder is a Zn-containingSn-based solder.
 8. A solder paste as claimed in claim 7 wherein theZn-containing Sn-based solder further contains Bi.
 9. A solder paste asclaimed in claim 6 wherein the isocyanuric compound is selected fromisocyanuric acid and an isocyanuric acid tris(haloalkyl) ester.
 10. Asolder paste as claimed in claim 6 wherein the rosin-based flux containsan activator.
 11. A solder paste as claimed in claim 1 wherein therosin-based flux contains an activator.
 12. A solder paste as claimed inclaim 11 wherein the activator comprises at least one organic aminehydrohalide.